texts: I have recently gotten into observing the night skies and learning about the stars/ star systems that are visible to me using an app on my phone. For the last month or so Venus and Mars appear in the early evening followed by some of the regular constellations (Orion, the two Canis'....) and later in the night skies Jupiter followed closely by the moon with Saturn peering out before daybreak. Yesterday night at around 20:15hrs I was looking up at the clear skies as I happened to be outside. I took a peek at the bright stars Sirius, Canopus and Rigel before concentrating on the Pleiades. As I was looking at that, one of the stars seemed to disappear in front my eyes. I thought it an illusion and carried on looking and about 6-7secs later a star appeared to the right of where I thought I saw one disappear initially. The light stayed for about 2secs and off it went again. I felt like someone was flashing a torch or even space station personnel having a call and response (baffled). One thing I was sure of was that it wasn't a plane as I am an Aeronautics man by profession and am familiar with all aircraft strobes. Another 6-7secs later it reappeared even further to the right than the second time round. For the next 11mins or so it came on and off at those regular intervals and yet still traveling to the right until it was out of my sight. tried to record a video of it with my phone but it wouldn't pick it up nor could it the other brighter stars. I was almost convinced by the end that it was some celestial matter but for the flashing light at exact intervals. Is it one of satellites in orbit? If so, is it normal for it to be equipped with flashing light/ strobe?


texts:
In the video Why NASA's SOFIA Telescope On A Plane is "Perfectly Balanced" after about 10:04 SOFIA associate director James Jackson says:

We have a spectrometer called GREAT, that’s our workhorse spectrometer that’s probably most popular. We have an imager called HAWC that does far infrared, and we have FORCAST, and we have another spectrometer EXES which is (for) shorter wavelengths.
[…]
Now… we’ve added a new wrinkle to our repertoire, we now have a polarimeter so we can measure the polarization of light, and that tells you how the magnetic fields are threading through these dust clouds or gas clouds. So HAWC does polarization as well, that’s a new capability
[…]
Well Orion for example… the galactic center… we’re looking at infrared… these snaky filamentary infrared clouds called infrared dark clouds. The basic question still is not answered because… does the magnetic field go “this way” with respect to the filament, or go “that way”; perpendicular, or parallel? What SOFIA is finding is that it’s more-or-less perpendicular, which is a surprise.

The text of the Wikipedia article Infrared dark cloud says (in its entirety):

An infrared dark cloud (IRDC) is a cold, dense region of a giant molecular cloud. They can be seen in silhouette against the bright diffuse mid-infrared emission from the galactic plane.
Infrared dark clouds have only been recently discovered in 1996 using the ISO and therefore are in need of further research.
Astronomers believe that they represent the earliest stage in the formation of high-mass stars and are therefore of great importance for understanding the star formation process as a whole.

That's not much to go on. This link is cited in the Wikipedia article and it says a little more in a descriptive manner:

Infrared dark clouds are cold, dense molecular clouds cores seen in silhouette against the bright diffuse mid-infrared emission of the Galactic plane.

and here's one of the images from that page:

8 micron image of G11.11-0.11 from MSX. It is seen in extinction against the bright diffuse mid-infrared emission of the Galactic plane.

The Wikipedia article also links to The early stages of star formation in Infrared Dark Clouds: characterizing the core dust properties but there's no mention of magnetic fields there either.
So I'd like to ask:
Question: What exactly are infrared dark clouds and what is the significance of the orientation of their magnetic fields? I'm also wondering if they produce these fields, or are passively aligned to existing fields.


texts: What’s the closest earth like planet we have discovered in our galaxy?

texts: What’s the closest earth like planet or moon to earth we have discovered in our galaxy?


texts: Pinhole projector for the Transit of Mercury

texts:
Very quick and simple one today. What would be the best/optimal pinhole size for a pinhole projector to observe the transit of Mercury on May 9th?

I want to get the optimum between resolution and brightness...has anyone any experience in this area?


texts: Could two stars collide without a resultant explosion?

texts: Are there any scenarios in which two stars could collide, and simply fuse without triggering a supernova. Maybe a mid-sized star slowly spiraling in together with a smaller star? It seems like any time stars collide it's incredibly violent. It seems that one star always gets torn to pieces and accretes matter onto the other, or ends in a red nova. Is there any way this process could be calmer?


texts: What is the difference between albedo, absolute magnitude or aparent magnitude? I can’t really tell what’s the difference between them.


texts: Do celestial bodies actually appear larger along the horizon?

texts:
Whether it be the moon (especially when full) or tonight, as Mars is closer than it has been in decades, it appears that these bodies are larger when close to the horizon than overhead.

Is this an optical illusion (i.e. the actual appearance is no larger), or is there some refraction or other effect that actually makes the appearance larger?


texts: how does Kepler stop exposure after a 5 minuate or 30 minute cadence time?

texts:
There are two kinds of cadence time for Kepler, 5 minutes and 30 minutes.
I just know there is no shutter and the CCDs of Kepler can be locked after exposure. handbook

But how to realize that? Remember we have to not affect the photometry.

The other question is so far how many unique sources has Kepler observed? 160,000?


texts: How do they measure sub-centimeter wave height on a moon of Saturn?

texts:
Michael Stevens from V-Sauce has a video called The Mandelbrot Set on a different channel. At 12:56 he says:

  There’s a moon of Saturn that has liquid hydrocarbon lakes that literally have waves that are only like a fraction of a centimeter large, but, you could surf that, right?

While Michael occasionally does indeed say things that appear to be wrong I'm guessing he got this correct.

How are waves a fraction of a centimeter tall measured on a moon of Saturn? How is the existence of these waves either measured or deduced?


texts: Question: calculating orbit using another orbit

texts:
A satellite which is moving around a planet of mass $M$ in a circular orbit of radius $R$ is given a sudden thrust into the center of the planet, so that it deviates an angle $a$.
Find the latus-rectum $l$ and the eccentricity $e$ of the new orbit.


texts: Looking at the big moons of the outer solar system a clear pattern can be seen. Thick icey surface layers covering rocky mantles and deep down metallic cores. The only outlier is Io as it has no icey, but a rocky surface. Did Io form this way or has Jupiter's tidal heating now powering it's intense vulcanic activity cooked away its oceans? After all water vapor could not be retained on Io so if the surface ice where to melt it would escape Io for good.


texts: Is the sun flipped on the other side of the world?

texts:
As the orientation of the moon is different in the northern and the southern hemisphere, is the orientation of the sun different in both hemisphere?

Does a sunspot appearing in the Sun northern hemisphere from the Earth northern hemisphere will look as located in the Sun southern hemisphere from the Earth southern hemisphere ?

If I understand well, an equatorial mount can compensate for the Earth tilt, but how is possible to compensate from one hemisphere to the other one? Is the image flipped?

thank you!


texts: Jupiter's magnetosphere interacts with the one from Saturn?

texts:
Jupiter's magnetosphere extends "seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction" and Saturn's magnetosphere extends towards the Sun on average 22 Rs (Rs=60,330 km is the equatorial radius of Saturn).
My question is: do these magnetospheres actually touch each other when the planets are on the same line?


texts: How was all the matter curled up inside a singularity during big bang ?

texts: If the singularity had infinite mass, temperature and density, then it should curve space and time but there was no space-time before big bang. So where did the singularity come to existence without space-time ?


texts: If the Oort cloud has billions of objects larger than 20 km, then why do we have so few comets?

texts:
According to Wikipedia:

“The Oort cloud may have billions of objects with absolute magnitude

brighter than 11 (corresponding to approximately 20-kilometre diameter)”


texts:
Imagine if Jupiter orbited the Sun at the Earth's orbital distance of 1AU.

Would a planet of this size orbiting the Sun take 1 Earth year (365 days) to complete an orbit, or would the size of Jupiter affect its rotation and orbit?


texts: What is the largest cluster of merging galaxies?

texts:
Reading about the Andromeda–Milky Way collision, a galactic collision predicted to occur in about 4.5 billion years between the two galaxies, I went to the galaxy merger page and Wikipedia states that :

  One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702.

The universe is a big place and wikipedia is not the most thorough with this subject.

What is the biggest merger we have found up till now?(29-09-2019)

Galaxy cluster MACS J0717, one of the most complex and distorted galaxy clusters known, is the site of a collision between four clusters. It is located about 5.4 billion light years away from Earth.
Image credit: X-ray: NASA/CXC/SAO/van Weeren et al.; Optical: NASA/STScI; Radio: NSF/NRAO/VLA


texts: Can we compress any object to create black Holes?

texts:
In general, when a star runs out of nuclear fuel, gravity gets the upper hand and the material in the core is compressed even further and creates black holes. I am clear till  here.
Now the question is: Does that depend on the mass of the star (or object)?
Naturally, the more massive the core of the star, the greater the force of gravity that compresses the material and easy to create Black Hole.
For smaller stars, when the nuclear fuel is exhausted and there are no more nuclear reactions to fight gravity, the repulsive forces among electrons within the star eventually create enough pressure to halt further gravitational collapse. The star then cools and dies peacefully.
In science videos and discussions is said that we can create black holes from any object. For example, if we are able to compress Earth to the size of the tennis ball, can we then create a black hole?
Isn't the nuclear fusion / reaction necessary to create a black hole? Of course, to fight against gravity we may require some kind of energy in this case, Nuclear reactions. But I am generally concerned like, does Nuclear Reaction is only to fight gravity, it does not play any other role in creating Black Hole.


texts: What do we know about the characteristics of stellar objects, 13,3 billion light years away from us?

texts: To receive light from objects like MACS0647-JD, I suppose, it has to has extraordinary dimensions and intensity of electromagnetic radiation. What are the mass, the dimensions and other characteristics for this objects?


texts:
The semi major axis (a) is not immediately visible with an hyperbolic trajectory but can be constructed as it is the distance from periapsis to the point where the two asymptotes cross. Usually, by convention, it is negative, to keep various equations consistent with elliptical orbits.

I was reading about hyperbolic orbit and found this in the wikipedia, what does it mean?
Can we just make a distance negative just to keep various equations consistent?
I am a high schooler beginning my prepartions for Astrophysics Olympiad so I don't have an in-depth understanding of central forces. Sorry if this question comes as if I didn't do enough research on the topic.


texts: From Mean Moon to True Moon in an old procedural calendar

texts:
This is a follow-up to How to interpret this old degree notation?

It is about an old calendar system which generates a luni-solar calendar with a procedural method, using a handful of calculations and some constants derived from astronomical observations.

This method originates from ancient India. Today it is used in Thailand to determine public holidays and observance days (uposatha days) for Buddhist monks, which fall on New- and Full Moon days.

Actually it's a neat system that had allowed village "astronomers / astrologers" construct a calendar with a method they could memorize by rote learning, without astronomical tools to make precise measurements. It is dreadfully underdocumented, and I've been collating the method and it's practices in Calculating the Uposatha Moondays.

Most useful have been the actual formulas in Rules for Interpolation in the Thai Calendar by J.C. Eade, it is just some basic arithmetic, but following his notation is a puzzle in itself.

Effectively I'm trying to implement these formulas (see image below), in a golang package.

With the earlier help, I arrived at step 13 (Mean Moon). Hooray :)

Step 14. and 15. is a puzzle again.

Step 14.

How would you interpret this? "(1780 + 80) * 3 on base 808, and add 2"

Step 15.

Eade has (8; 11 : 7) - (6; 27 : 12) = (1; 3 : 55), but that doesn't work.

That specific substr. gives me (1; 13 : 55), which could be a typo in the paper, but that value doesn't carry things forward either.

Things straighten up at step 16., 17., 18., which would produce the paper's results, to get there I faked the values for step 14. and 15, to work at least in the example case given in the paper.

In suriya.go
this business is happening in:

func (suDay *SuriyaDay) Init(ce_year int, lunar_year_day int)


texts:
Is it a coincidence that Earth's axial tilt and maximum declination are both approximately 23 degrees?

Mechanically speaking, do planets physically cross the Solar equator, or is declination due to the axial tilt angle? The later is difficult to visualize given variety of planetary axial tilts i.e Mercury 0, Jupiter 4, Earth and Saturn 23 degrees approximately.

I'll try and give a concise example. At JPL's Horizons you can download data of RA and DEC for all planets and objects placing either Earth or the Sun at the center. This will calculate the Declination of a planet geocentrically or heliocentrically.

The calculations suggest all planets will obtain a maximum declination North and South between 23° and 28° (i.e Mercury reaches 28° when viewed from the Earth). Earth's axial tilt (obliquity) at 23.45° matches its maximum declination N/S.

However, Mercury's Axial tilt is 0° and Jupiter 3.13° so if declination is the angle between the ecliptic plane and the celestial equator how do Mercury and Jupiter obtain maximum declination of 23° and 28° N/S?

I'll appreciate any help clarifying this.


texts:
I came across this sentence in a magazine:

  We sit outside the Lorne Community Hall, Victoria, watching the MIA satellite overhead.

I wonder if the author is talking about an artificial satellite. Can one recognize an artificial satellite with naked eyes?

Thanks for response.


texts:
Can you explain what is a Friedmann model to a layman?
And also give some examples of Friedmann models, specially I would like to know if the lambda-CDM model is considered a Friedmann model.


texts: 60 second exposure photo with phone at eyepiece with an app produced giant ball of light, what next?

texts:
Last night I tried my first long exposure photo of M44 (the beehive cluster). While my exposure time was 60 seconds, my photos came out with a giant ball of light in the center of the photo.

Could this be due to my set up, as I only was using a phone app and putting my phone up to the eyepiece for one minute?


texts: What are the experimental limits to the residual charge of the Sun?

texts:
The question Can a Neutron Star become charged? and the associated comments and answers started me wondering; Are there measurements of, or experimental limits to the residual charge of the Sun?

Because the Sun has a complex atmosphere and a net solar wind, I there may be some subtleties related to what sphere is used to establish the net charge Q inside, so rather than me define a radius it may be better to see what evidence is available.

Since there is an ionized component of the solar wind and protons are 2,000 times heavier than electrons, I'm thinking that an initially neutral Sun would loose electrons more rapidly than protons, until a strong enough static field was produced that would add the extra "push" for the protons to leave at the same rate, but that's an extremely simplistic model.

I'm asking primarily for some kind of measurement or experimental data rather than purely rationalization or hand-waving. Has there ever been an attempted measurement of the static residual charge of the Sun?


texts:
I am trying to figure out the position of sun with respect to moon on a full moon day.

Here's an image from certain wiki which I do not find convincing.
1

If the sun rays point as shown in the image and the earth stands between the moon and sun on Full moon's day, I dont see any reason why the moon should be shining in its full glory.

Similarly when the moon is directly in between earth and sun, it should have maximum brightness but the image shows it as the new moon.

In fact, I cannot deduce how at all can moon receive light across the entire sphere at any given angle from the sun.

Putting it specifically, what would the position of sun w.r.t to earth and moon on a full moon day ?


texts: Declination and Ascension - the Sun and Andromeda

texts:
If Andromeda is overhead at midnight, what is the right ascension of the sun? (andromeda is ar RA =1hr, Dec = +40).

I thought I understood what Dec and RA were, but I can't think of how to start this question?

Thanks.


texts: How does the geometry for constructing a declining vertical sundial work?

texts: I understand how to construct South-facing vertical an horizontal sundials and the geometry behind their construction by projecting an equatorial sundial into a plane, but I can't quite get how to do that for a declined plane.


texts: How to calculate geocentric conjunction time and moon altitude at a given time

texts:
Umm al-Qura calendar of Saudi Arabia determine the new moon if these criteria meet at 29th day:

The geocentric conjunction occurs before sunset.
The moon sets after the sun.

I've got the formula to calculate when the sun (seen by the observer) sets, considering observer's latitude, longitude, height (altitude from sea level), and the date (Julian Day). I would like to ask the formula to calculate:

When (the time) geocentric conjunction happens.
How much (the angle) the altitude of the moon (seen by the observer) at a given time is.

Thank you


texts: minimum size for a black hole to capture a photon

texts: If a photon is moving past a black-hole, what is the minimum size of the black-hole that will capture the photon and prevent the photon from going onto orbit around the mass? Do black-holes exist that have a stream of photons orbiting them?


texts: How do we know if an object is redshifted?

texts:
I have some questions about redshift. I think that the first two hang on the last two, but feel free to point out anything you think I should know.

How do we know if an object is actually redshifted, and that we aren't just seeing the regular spectrum of the star?

How can we tell the extent to which a star is redshifted?

Do astronomers have a full baseline spectrum that they compare the measured spectrum against? Or do they just use the hydrogen spectrum since all stars are mostly hydrogen?


texts: The life course for a massive star from birth to death using the HR Diagram

texts: Could you explain to me the life course for a massive star (30-40 solar masses) from birth to death using the HR Diagram (by showing key events in its life)?


texts:
When looking into the sky at night using my bare eyes, I see that stars appear in different colors. From my understanding this is caused by different chemical compositions of those stars which show up in their spectral lines and therefore defining their color.

Does blue and red shift visibly change the color perception when looked at the stars with your naked eye?


texts: How the depth[Mpc] is obtained from the redshift

texts:
I read this paper.  In this paper, they show the three-dimensional distribution of the [OII] emitters.
It is mentioned that The depth in z-axis is converted from the redshift, which is estimated from the ratio of [O II] emission lines measured with NB912 filter to that measured with NB921 filter. This is shown in this fig

I have this doubt that how to get this depth in Mpc from the redshift. They mentioned that the centre of galaxy cluster at z=1.5.


texts: Is the mechanism of solar flares on red dwarfs and brown dwarfs the same as that on the Sun?

texts:
The Sun has solar flares that are caused when there is a magnetic reconnection in the Sun's atmosphere, causing a loop of magnetic field to be ejected at high energy, along with a large number of protons, giving a intense burst of radiation.

Other stars - notably some red dwarfs - have much more intense flares than the Sun does. Is the mechanism the same in these stars as in the Sun, and in larger stars? What causes the flares in red dwarfs, and even brown dwarfs? Why do less powerful stars have more powerful flares?

Jupiter also has a magnetic field, and it's magnetosphere is composed of plasmas. Why aren't "Jovian Flares" seen?

Related: How do large solar flares compare to flares on other stars?


texts: What is the direction of the movement of the solar system in relation to the galaxy's plane

texts:
I got from this answer that the axis of the solar system is tilted of about 63 degrees in relation to that of our galaxy, so I can assume that we move through space (at least locally) roughly in a direction that is 63° tilted in relation to our solar system's plane, is that correct?

If so, then: is there also some net velocity of our solar system (probably small) in a direction that is perpendicular to the galaxy's plane, as opposed to the main direction that is "parallel" to the galaxy's plane?

Thank you in advance.

EDIT

I think my question is different from this one in that the latter asks about the distance from the solar system to the galactic plane, with no regards as to the speed with which this distance is changing. On the other hand, I'm interested not on the distance, but on the speed with which this distance is changing, which can promptly be approximated with respect to time with a sine function from pela's answer, which was my main intention.


texts: Pictures of a curious astronomical phenomenon

texts:
In a recent holiday, a friend made the following picture. It contains a curious green "thing" in the nightsky. The picture was made in Ladakh, India, 6weeks ago.

The "thing" only appeared in one of the pictures (others were made a few minutes earlier and later). He didn't see it with the naked eye (but didnt look at the sky during the exposure).

Camera: Canon EOS 350D DIGITAL
7 sec exposure
ISO: 1600
f-number: 4

What could that be?


texts: How fast does the blast front of a supernova expand at? Is it close to the speed of light or is it less than a quarter of the speed of light?


texts: Contradict the expansion of the universe?

texts:
Lets say that we observe in a distant galaxy cluster, which in all the galaxies move far away from us.
and here we see in the same cluster a galaxy that move away from us in a much lower velocity than the other galaxies.
is this contradict the expansion of the universe?


texts: How, exactly, does the precise measurement of the CMB's polarization modes and temperature fluctuations tell us the value of Hubble's 'constant'?

texts:
Amidst all the talk a year and change ago about the value of the Hubble parameter reached by the Planck satellite team, and how it's value differed from the value reached by the 'distance-ladder' team(s), I've yet to read an explanation of how precise measurement of the cosmic microwave background's 'temperature' fluctuations and E-mode polarization(s) ultimately give you a value for Hubble's 'constant'...
I presume the answer is a bit complicated, or I probably would have found it somewhere...
Also, the Planck satellite shut down or whatever in 2013, and this news came out only last year...


texts: Does anyone out there have a script to replicate the star of Bethlehem.  In this case meaning a conjunction of Venus and Jupiter near Regulus.


texts:
Sunspots, such as this one, appear dark:

Why?


texts: Is it correct to say vertical conjunction?

texts:
This question is related to this one: Are we seeing a conjunction these mornings?

Last Feb 27, the a reporter said that, the day before, at sunset, Venus and the Moon were in vertical conjunction. Is that correct?

Sorry, but I cannot understand the meaning of near conjunction.


texts: Can we observe changes in the CMB (surface of last scattering) over time?

texts:
It is hard to determine the age of the Sun, because it won't change much over 100 million years or so. But what about the cosmic microwave background radiation, that is being mapped with ever better precision. It dates to only about 380 000 years after Big Bang. Is it possible to observe changes in it over a period of 10 years or 100 years, as it ages and cools, as the Hubble Bubble expands?

And how is the landscape of the CMB-map expected to change? Are relatively cool and hot areas permanent over time scale much larger than 380 000 years, or would our CMB-map be unrecognizable if we had lived, say, a billion years earlier or later?


texts: Can you recommend a book about big bang nucleosynthesis and chemical abundances?

texts: I am interested in learning about big bang nucleosynthesis, nuclear fusion up to iron in stellar cores and beyond iron in supernovas, and into the lithium problem (galactic abundance anomoly for atomic numbers 3,4,5). As these topics are interlinked, I have been hoping to find good resources on the subject. Can anyone recommend a good source for self-study? In case it's relevant (regarding difficulty level), I'm a physics grad student lacking exposure to related topics.


texts: How did the Earth come to be in orbit around the sun?

texts:
I'm not sure if this post should be in the physics subject forum, but this seems to fit here too. I have been reading a book about chemistry and how the universe came to be with the theory of stars creating most of the elements we know of. I began wondering how the Earth came to be in orbit around the Sun. I understand that we are in a constant free fall towards the Sun, but our motion causes us to "miss" the Sun stopping us from burning up. However, I'm confused how we were put into motion in the first place. My best guess is that we were captured by the Sun's gravity, and were put into orbit, which would mean the Earth was once a meteor, right? If someone could please explain this, it'd be very helpful because I can't seem to find the answer anywhere.

Thank you

P.S. I'm just a 9th grader, so I don't have much knowledge in astrophysics.


texts:
My question concerns the following problem, published in Galactic Dynamics by Binney and Tremaine and based on a work by Schwarzschild.
Specifically, I am having difficulties in deriving the given result in part (c), which I suppose is due to the confusion surrounding the definition of strip brightness. Could someone please explain, with the aid of a diagram if possible, how $x$ is related to the projection, $R$, of the  radius $r$ onto the plane of the sky? I am having trouble visualising what $x$ is measuring here.


texts: Is it possible for galaxy clusters to interact?

texts: Galaxies commonly interact and collide with other galaxies. Is it possible that clusters of galaxies similarly interact and collide? Have people tried to study this phenomenon previously? I couldn't find anything specific in the literature.


texts: Data on the composition and orbits of asteroids

texts:
I am interested in the composition of asteroids from the perspective of determining potential targets for mining.

I expect there already exists some data that can be used to identify reasonable targets for mining.

The specific mining method I am looking to evaluate is to send a "collector" out and "grab" a target, bringing it back into LEO for mineral extraction.  Because of this I would like to identify targets that are < 20m diameter. The ideal material to mine/extract would be a precious metal of some kind. This would give the most "bang for the buck". (The buck in this case being the effort rather than Delta V).
Considering the ideal material, I suspect a list of "M" type asteroids and their properties would be the data I am after.

I saw a question that asked about using IR Spectroscopy to determine the surface composition of asteroids.  That gave some good insight - but no indication of where such data currently exists. A table of this data would be a reasonable starting point.

I am also wondering if we can determine anything about the relative density of an asteroid by the gravity it exerts on nearby objects.  If that data is available anywhere that would also be an interesting place for me to look.

I am also interested in trying to determine the orbits of any target asteroids, so where would I look for data on the orbits of those that are selected?


texts: Will new stars stop forming at some point of time?

texts:
New stars keep forming in the universe thanks to all the nebulae. Now, we need Hydrogen to form stars and there would a time when all the hydrogen will get exhausted, and no more star formation will take place, theoretically.

Will there practically be such a point of time? I guess there is no place where hydrogen is replenished after star formation. Or is there a feedback cycle?


texts: What orbit is the smallest or takes the least time to complete?

texts:
What orbit in our solar system would take the least time to perform 1 orbit?

What is the fastest possible orbit to complete in the universe?


texts: Why does the half moon appear to have a "nose"?

texts:
Yesterday evening, the moon from my place looked like this:

Basically, instead of the crescent shape I am used to it looked like there was three tips: one at each extremity (as usual) and one in the middle between the two first ones. This picture shows the rough shape of the visible moon, with arrows pointing at all three tips:

What is the explanation of this?


texts:
Days before, I saw this pic. I don't know what it is.

Edit: This pic was taken by a Taoist at an ancient ruins in Mongolia on 2012-07-15 04:12 UTC+8 (day break). He said it looked strange.

Edit: I installed Stellarium, changed the time to 2012-07-15 04:12 UTC+8, and faced east. It seems to be the Moon.


texts: Are there known centaurs which orbit between Jupiter and Saturn without crossing their orbits?

texts:
According to Centaur minor planet

  Centaurs are small Solar System bodies with a semi-major axis between those of the outer planets. They generally have unstable orbits because they cross or have crossed the orbits of one or more of the giant planets

Of all the known ones which I've found and I've seen data, all of them overlaps the orbit of Saturn, with the exception of  (514107) 2015 BZ509 which doesnt overlap the orbit of Saturn but the orbit of Jupiter.

My question is, are there known centaurs which dont overlap the orbits of Jupiter and Saturn but which orbit between those 2 orbits? Or does the definition of Centaur mean they have to cross orbits and an asteroid orbiting between those 2 orbits would be called another name if exists?


texts: Is the angular size of the black hole in the movie "interstellar" completely overblown?

texts:
I assume everybody is tired of reading questions derived from the movie Interstellar, I will try to keep this short and simple:
In general in movies, in order to have stunning visuals, celestial objects are displayed as if they were extremely close to the observers, having a enormous apparent angular size.
One striking example of this that has seems counter-intuitive, is the angular size displayed for the black hole "Gargantua" in the movie Interstellar. The characters in the movie are so close to it that it can be fully displayed in great detail (see attached image: Black hole seems to cover almost/over 90 degrees of the field of view..!)
Now, we can fully experience gravitational pulls from much smaller objects than a supermassive black hole in reality. The angular size of the sun and the moon are so small that we need telescopes to appreciate details, despite their gravitational effects dominating our everyday life.
It seems to me that a supermassive black hole would have such immense gravitational effects, that in order to be able to "freely" navigate around it (as done in the movie), observers would be so far away that the angular size of the black hole would be tiny (which would result in very non-spectacular cinematography).
The question is: Realistically, for a supermassive black hole ("with a mass of 100 million times of the sun"), what would be roughly its angular size when standing at a "safe" distance from it?


texts: Why didn't Johannes Kepler use data about more planets?

texts:
In Astronomia Nova (1609) Johannes Kepler used observations of Mars in order to refute circular orbits. Throughout Astronomia Nova he hardly even mentions other planets than Mars, Earth and the Sun (and the Moon in a different context). Were there any astronomical reasons for him not to continue, or even beginning, his investigation by using data about other planets?

Tycho Brahe had collected corresponding data about the other planets too, right? 20 years of data is enough for Saturn to make 2/3 of its period. Jupiter and Saturn have almost twice as frequent oppositions as does Mars. And Mercury has twice the eccentricity and its much shorter period should help getting data about its apsides when it is best seen.

Would it had helped him to use several planets during the process as he describes it in Astronomia Nova?
Was it simply the calculation capacity which limited him from dragging several planets into it?
Or could he somehow conclude that it is enough to solve for Mars, to know that his physical conclusions of interests are valid for all planets?


texts: How big would the supernova when a neutron star collides with another neutron star?

texts:
If two neutrons stars that are orbiting were to collide, how big would the supernova explosion be?
Would a new black hole or a new supernova remnant be formed?


texts: How do we have photos of galaxies so far away?

texts:
A possible answer for this is that, light emitted from the galaxies travelled a billion miles all the way to earth, where the hubble space telescope picked up this light through its sensors, and was able to construct an image of the galaxy

but if this is true, and galaxies are billions of miles away, shouldn't the light particles emitted from the galaxies be scattered all over the place? after all they have been travelling from millions of years, and have probably collided with asteroids and other foreign objects. What were the chances that about 95% of the photons actually reached earth, giving us a very detailed image.

Consider the andromeda galaxy which has a distance of 1.492 × 10^19 mi from earth. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy, evident from the photo below?

Shouldn't like half of the galaxy be missing since photons could have hit other objects, and "never have reached earth"?


texts: When are wedge prisms used to correct chromatic effects of atmospheric refraction?

texts:
Atmospheric refraction (shown below) happens because Earth's atmosphere has an index of refraction that differs from unity.

@MikeG's comment mentions that this refraction would have a chromatic component (since the index of air varies with wavelength) and that observers sometimes use a wedge prism to correct for it.

I suppose it would be more important for a wide spectrum image than for narrow band imaging.

How often is this done in practice these days?
How often was this done in the past with emulsion rather than CCDs?
Are there any notable cases or observations where this is/was very important?
Roughly how strong is the effect? If the average refraction is 2 arcminutes, roughly how many arcminutes would a glass wedge need to be to correct the chromatic aberation of the atmosphere?

Plot of atmospheric refraction vs. apparent altitude, using G.G. Bennett’s 1982 formula. Author: Jeff Conrad


texts: Does a gravitational lens around a body look 3-dimensional?

texts: Suppose we approach a neutron star or a stellar black hole close enough, is the gravitational lens around it 3-dimensional (like a tunnel) or is it 2-dimensional? It's 3-dimensional as if you entered a spacetime tunnel, isn't it? If you entered the tunnel, sidewards you'd see the firmament distorted too, right?


texts:
My question is this : could the Charon-Pluto system becomes in the coming few millions years a full fledged planet, as in the IAU classification ?

I ask this because there are two facts that bother me :

The system orbit isn't on the ecliptic
The system hasn't cleared its neighborhood

So, is it possible that that system isn't stabilized yet, and that 1) its orbit is very slowly changing (for lack of a better word) to the ecliptic, and 2) that the system will slowly collapse, and bring all its moons with it, to clear its neighborhood ?

In other words, is Pluto a "proto-planet" (as I would personally name it), or definitely (and for the next millions years) a dwarf planet ?

Edit : I disagree on the fact that this question is a duplicate of How long will it take Pluto to grow to planet size? as this one ask not for the size of Pluto, but rather the other characteristics, the orbit and the clearing the neighborhood.
Moreover, the question was asked in a broader canvas, to know if it was possible that some elements of the solar system (Pluto, but not only) could be instable enough to reach the level of planet sometime in the futur.


texts: How do we distinguish old craters from new ones on the Moon?

texts:
The following image is the observation made by the Dual Frequency Synthetic Aperture Radar (DF-SAR) onboard the Chandrayaan-2 Orbiter:

Due to image size restriction, a lower resolution image is used here. In case you wish to see the original image, visit this ISRO Webpage.

In the above image, there are three craters marked Young (Fresh), Intermediate and Old, and we are able to see the difference clearly. This ISRO Webpage states that the difference is due to weathering processes. I can't understand how can such processes undergo in our Moon, where there is no appreciable amount of atmosphere. Is that because of Moonquakes?

It would be great if you could explain this.


texts: Is Earth slowing down in order to stay on its orbital track?

texts:
The Sun is losing some of its mass during explosions.
The Gravitational force between Sun and Earth will change (probably decrease).
If Earth maintains its speed, it will change its orbit.
Hence, Earth will decelerate

Is this true?


texts: extracting visibility information from CASA measurement set

texts:
I am trying to export the UV wavelengths, visibilities and weights from a measurement set in CASA, but I haven't been able to find the right commands to do this.
'exportuvfits' might be what I am looking for, but I can't find a clear explanation of what this format really is.
Any CASA users out there that can help me?


texts: Why could a gas planet not get much larger than Jupiter?

texts: What is the maximum size for a gas giant?


texts:
Does anyone else watch 'Dr. Becky' on YouTube?
Her March 'Night Sky News' video, subtitled 'The biggest black hole burp, a wobbly Milky Way & Betelgeuse is brighter' says that...
"Dust and magnetic fields are the bane of astronomers".
Why?  How do magnetic fields mess with astronomers' observations?
https://youtu.be/1cw2jJv3Wr8?t=565


texts: If space is undergoing accelerated expansion then regions of space are becoming inaccessible to us even at light speed. If gravity is bounded by light speed then inaccessible regions of space no longer contribute gravitational influence on us. If true wouldn't gravity waves... tiny variations in gravity given sensitive enough equipment... be all around us as matter crosses the threshold and becomes inaccessible to us? Does this violate conservation?


texts: time on earth compared to completely stationary in space

texts: I recently started trying to understand the theory of relativity and I have the following question.  Because earth is orbiting the sun at approx 67,000 mph (and its my understanding time slows down the faster you are moving) what are the effect of time on earth compared to a completely stationary observer watching earth's orbit, from outer space?  How would the twin paradox work with one twin on earth and the other twin completely stationary in outer space?


texts:
Now that the Gaia Space Telescope is on it's way to the Sun-Earth L2 Lagrangian point (SEL2), I start wondering about the stability of Gaia's orbit there. The Planck Telescope is already there, as was Wilkinson Microwave Anisotropy Probe (WMAP) and other probes, and from Wikipedia I learned that:

  In practice, any orbit around Lagrangian points L1, L2, or L3 is
  dynamically unstable, meaning small departures from equilibrium grow
  exponentially over time.

Gaia has some kind of Orbital Maneuvering System (to borrow a Space Shuttle term) and some propellant on board, so has Planck, however I wonder how deterministic these orbits are and if both Planck and Gaia have automatic corrections and collision detection in their flight computers; L2 is "only" 1.5 million km (or about 5 light-seconds) away so surely there's time for manual correction.

Does anyone know a source which tells how different Gaia's and Planck's orbits are, if there are intersections between their orbital planes or even how likely the need for an unplanned orbital correction is? I know Lissajou-shapes from maths classes and I know how much the projected trail can differ depending on the precision of data types used in calculations (e.g. float vs. double). How does ESA/NASA handle this, now that SEL2 seems it will become a crowded place?


texts: What does designation VY, NML, UU in star names stand for?

texts:
I've read the a IAU page on the subject but i am no closer to understanding how this works. Also did search the already asked question, but didn't see anything on the subject.

What does bolded item in the names below stand for? How do they choose them?

VY Canis Majoris
NML Cygni
UY Scuti


texts: Moving lights in the sky looks like they were moving coordinated

texts:
I was out with a couple friends a night and we looked up to the sky just watching the stars. I noticed what looked like a star moving then told my friends and they saw it too.

After we saw the one moving star we noticed 2 more, it seriously looked like 3 stars just decided to have a meeting. It was the weirdest thing I've ever seen


texts: Is Optical VLBI theoretically feasible? If not why not?

texts:
There are plenty of optical interferometers in use with baselines of up to maybe 1km. As far as I can find out, they all work by directly collecting the light at all the telescopes, using mirrors to bring them together and then actually interfering them and observing the fringes with photodetectors.

Meanwhile, radio astronomers do interferometry over distances of thousands of km, but they do it quite differently. Essentially, they record the baseband electromagnetic waveform at each telescope, with an accurate timestamp, transport them all to a central location and combine them computationally.

So my question is whether in principle you could use the radio astronomers methods with an optical (or near IR) signal. Obviously the engineering and computational challenges would be formidable -- you would need to record data at speeds close to 1 Pb/s, timestamp it with femtosecond accuracy and do a ludicrously huge computation to recover an image, but that's not my question here.

On the one hand, it seems like the answer should be "yes" -- electromagnetic waves are electromagnetic waves. On the other hand, why should the phase of a photon observed in one telescope have anything to do with the phase of one observed in another? If you are digitising the data at each telescope, you are ruling out interference  fringes on a quantum level, just as happens if you find a way to measure which slit a photon went through in the double-slit interferometer. This suggests that the answer should be "no".

If the answer is "no" then what wavelength does the cutoff happen at? We have radio-style VLBI at 1mm now and optical-style interferometry at 1 $\small\mathsf{\mu m}$, what is possible in between?


texts: Find x,y,z velocity component of a star?

texts: I am trying to find the initial x,y,z velocity components of the Epsilon Lyrae stars. How might I go about doing this? The radial velocities are available on Wikipedia, but I am not sure how to use that data for my purpose.


texts:
do you know any website in which I could obtain orbital elements of planets at some date?

For instance, I'm now needing the orbital elements of Mars around June, 2014.

Thanks!


texts:
I'm really confused with this one. Almost all documentation says you must start with your telescope pointing to the Celestial Pole and for the lucky ones who live in the north you can use Polaris.

But I live in the Southern hemisphere (Chile), so besides no Polaris for me, the real question is: Should I start with the telescope pointing to the South or North Celestial Pole?

I have had a month of frustration trying to align.

I have looked into other answers here and other sites, but no one explicitly says where to point it and how to find it.


texts: How can black holes be sometimes so gaseous?

texts:
One of the most massive Supermassive Black Hole observed is the one at the centre of the galaxy NGC1600 with a mass of 17 billion suns. It would have a density of ∼0.01kg/m^3, or one part in 100,000 times the density of water, or 1% the density of earth atmosphere at sea level.

But if there is so much gas why isn't the bh shrinking and becoming more dense as gravity would pull everything together?


texts: Are there recommendations to use "Hubble-Lemaître constant" instead of "Hubble constant"?

texts:
The IAU recommends renaming the Hubble law as the Hubble-Lemaître law, as discussed at its General Assembly in 2018 (see this press release and the links therein).

I have now stumbled upon an occurrence of "Hubble-Lemaître constant". Are there any recommendations (or noteworthy statements, opinion papers) in favor or against this extended interpretation of the IAU resolution B4 to the terminology for $H_0$?


texts: Do hypervelocity stars have Oort clouds?

texts: If you had a hypervelocity star travelling through space incredibly quickly, would the hypervelocity star's Oort cloud remain intact? Would the high speed of a hypervelocity star shed the comets and dwarf planets in its Oort cloud due to the star's high speeds, or would the Oort cloud get stretched into a teardrop shape? Could the Oort cloud remain unaffected?


texts:
I was looking at the Jupiter for the first time today and I have noticed a smudge near it. Could someone explain what it is, please? I was looking online but did not find anything. So I thought that some kind of defect in my setup but I couldn't get rid of it. I mean the thing below its moons.

Edit1:
My telescope is a Celestron AstroMaster 76 ESQ. The smudge was there no matter what eyepiece I have used. I was able to see it even without a camera.

Edit2:
I was watching it for about 10 minutes, and the smudge was moving along with the rest of the objects.

I took just this one image for the StackExchange purposes. I have put the camera in front of an eyepiece to take the picture.

Thank you


texts: How to compare a reflector telescope to a refractor one?

texts:
I already have a small 60mm Tasco refractor (with 300mm focal lenght) and a homemade 40mm refractor (with 1300mm focal lenght) and I want to buy a new telescope to have a better experience (I don't have an EQ mount yet)

I face the current choice: for the same price, I can have a 70mm refractor (900mm fl) with a great EQ mount, or a 114mm reflector (900mm fl) with a less great EQ mount...

How can I compare those 2 two? They are not the same type, and I feel like comparing oranges with apples.

I know the technical difference between them, and I know the details about collimation. I just don't know how to rate them and chose one.

In short: Is it better a big reflector or a small refractor?
(If I want to extend the information to other choices I might encounter)


texts: Samples from the double Schechter luminosity funcion

texts:
I want to produce samples from a double Schechter luminosity function.
Ideally, I am looking for a python implementation of this problem but am open to other tools if necessary.
Do you know of any such package that will facilitate this?


texts: What object in the universe is most opaque to neutrinos?

texts:
I had this thought, and my first guess was "high density = lots of absorption, so I guess it's neutron stars" but this physics.se question about that has a great answer which covers why that's incorrect.

So what object will absorb the highest fraction of neutrinos passing through it, or at least be a good candidate? Feel free to assume a certain energy range of neutrinos. Exclude black holes because they just absorb everything and that's not as interesting.


texts: What is the minimum size to be a natural satellite?

texts:
Would a smaller satellite be more stable around a smaller planet where the same satellite would not be stable or last around a larger planet?

I added the picture to show how moon size planets can have even smaller moons.


texts:
Gravity is sometimes described as a curvature in space-time. Due to relativity, doesn't this imply that gravity doesn't propagate?

A thought experiment. If you are moving toward a black hole, this could also be considered as if the black hole is moving toward you. If a black hole was moving toward you at the speed of light, and gravity propagates at the speed of light, you would experience something similar to a supersonic blast, except its a gravity blast, and it would allow you to remain unaffected until you arrived at the very centre of the black hole?

If you were able to turn around, just before you arrive at the center, and travel away from the black hole at the speed of light you would never notice it, even though you would be travelling at the same velocity as the black hole in the same direction?


texts: What does observational evidence say about the number of protoplanets believed to exist in the early solar system?

texts:
Currently, the solar system has 8 major planets, a handful of dwarf planets and a plethora of asteroids, comets and natural debris, and of course, a rather locally dominant star (the Sun).  There are many references that describe the early solar system as a chaotic environment with many more protoplanets.

Based on observational evidence, how many protoplanets are believed to have existed in the early solar system?


texts: How do we know that an exoplanet is tidally locked to the parent star?

texts:
Often in discussing exoplanets we hear that they are close to the parent star (usually a red dwarf) but then there's a caveat that the planet is tidally locked to that star and that that fact severely decreases prospects for that planet to harbor life.

But how do we know that an exoplanet is tidally locked to a star? I doubt that we can directly observe the rotation of that planet. Does that mean that we conclude that it's tidally locked because that's what our theoretical models tell us, and not that we observe it directly?


texts: Assuming proton decay is false, what's the ultimate fate of a neutron star or white dwarf?

texts: A still unproven theory that protons can decay, and have a halflife of $10^{30}$ years or so, meaning eventually all matter will dissolve because their constituent protons and therefore neutrons will decay. But this is yet to be proven. So assuming this is false, what would be the ultimate fate of a neutron star or white dwarf? What would become of it in $10^{50}$, $10^{100}$ years?


texts: How could a 20 inch space telescope "be able to make out Earth-size planets" orbiting Alpha Centauri?

texts:
The NY Times article Telescope to Seek Earthlike Planet in Alpha Centauri System describes a proposed effort to put a telescope into orbit fairly quickly with a narrowly defined purpose - to look for planets in the Alpha Centauri system, which is about 4.37 lightyears away from Earth.
The funding, design, assembly and space deployment fairly quickly is made possible in part by the narrowly defined purpose and the compelling public interest in this particular question.
I got out my box of envelope-backs and did the following. Since there is no air, there's no need for adaptive optics so  the standard diffraction limit applies. For an Airy disk:
$$ \theta \approx 1.22 \frac{\lambda}{d} $$
The article describes the telescope as washing machine-sized, with an aperture of 20 inches, or about 50 cm.  If I choose a visible wavelength of 500nm, I get a resolution of about 1.22 microradians ($\mu$R). 4.37 lightyears is about 4.13E+13 km or 2.76E+05 AU.
That makes the airy disk about 0.34 AU at Alpha Centauri. The actual point spread function of a telescope depends not only on the theoretical diffraction of the aperture, but diffraction from secondaries and supports within the aperture, on aberrations due to imperfectly shaped optical surfaces and scatter and haze due to nano-roughness of mirror surfaces. (see below)
Could a 20 inch telescope and associated experiment package be put together quickly that could actually resolve (or "make out" as the article says) an Earth-like planet in the Alpha Centauri system from space? If so, what is it exactly that it could do that can't be done from any existing telescope in orbit or on Earth?

above: "A rendering of the compact exoplanet imaging telescope Project Blue plans to launch into orbit and aim at Alpha Centauri. Credit Project Blue" from here.
It jumps out that the secondary is mounted off-axis, which can have mechanical and optical advantages (e.g. diffraction) but would probably limit the image quality to a narrow field of view, which might be OK for a single-target telescope if there are suitable calibration targets nearby.

A nice discussion of the point spread function of the Hubble space telescope can be found here: 20 years of Hubble Space Telescope optical modeling using Tiny Tim (Krist et al 2011, Proc. of SPIE Vol. 8127 81270J-1), and the reduction of scatter and haze from mirror nano-roughness by using refractive optics (lenses) is addressed by the DRAGONFLY instrument. See also What (actually) is the " deprojected half-light radius" of this almost-all-dark-matter Galaxy?


texts: Why does the sun rise north of east between the vernal and autumnal equinox?

texts: Can someone please explain this phenomenon? A solid explanation seems to be missing from the entire internet.


texts: What will the last day of humanity look like?

texts:
It's certain that human life on Earth will not go forever. Therefore there will be one last generation of humans on Earth.

How will life look like for last few generations approaching the end?
How will life end for the very last people, will they all die at once as if struck?

Assume the technology won't be much more advanced than today, no colonized planets, and we'd stay biologically very similar to now. Also assume that no meteor will struck us and end everything but that extinction will happen due to aging of the Earth and Sun.


texts: Usage of $\sim$, $\approx$, $\simeq$, and $\cong$ in observational astronomy?

texts:
My understanding is $\sim$ generally means "on the order of magnitude of" e.g. $T \sim 10^5$ K

$\approx$ is obviously "approximately equal to" so for example one might write $d \approx 400$ pc rather than $d=4 \times 10^2$ pc

$\simeq$ and $\cong$, are where I am more confused as their usage is rarer and less consistent.

The below IAU resolution, for example, seems to use $\simeq$ for truncation $m_{\mbox{bol}} = -26.83199\ldots \simeq -26.832$, but I have also seen $\simeq$ used as "approximately equal to" in other works.

http://arxiv.org/pdf/1510.06262

How should one be using $\sim$, $\approx$, $\simeq$, and $\cong$?


texts: Has the inclination of the orbital plane of Phobos and Deimos followed that of Mars' axial tilt?

texts:
Phobos and Deimos are very close to Mars' equatorial plane (the inclination of their orbits is roughly 1°). The axial tilt of Mars has changed a lot over time, and might have had a inclination of over 60° compared to the ecliptic at times.
Did the inclination of Phobos and Deimos also change over time, following the axial tilt of Mars (and if so, by what mechanism?)? Or have the equator of Mars and the orbits of Phobos and Deimos lined up at the present time by chance?


texts: Is HR 5171 a Spectroscopic Binary or a Symbiotic?

texts:
Is HR 5171, the largest yellow star in the galaxy a symbiotic star where one feeds off the other? Or is it a spectroscopic binary where its only possible to determine that there are two stars by spectral analysis?
Will the smaller be consumed by the larger one as was in the possible case of BP Piscium? Or is the smaller one being born?
Is the star both, one or the other or neither?
I looked at Simbad but it only calls a Double or Multiple Star System. That is too vague for me.  Calling it a Close Contact Binary is also too vague.  I know they call it an EBS.
If anyone wants an idea of the star, visit the following artist's impression.


texts: Strange matter objects other than neutron stars?

texts:
This question is about celestrial objects which are composed by exotic matter, and exotic matter I define as

states of matter that are not commonly encountered such as Bose–Einstein condensates, fermionic condensates, nuclear matter, quantum spin liquid, string-net liquid, supercritical fluid, color-glass condensate, quark–gluon plasma, [...] but whose properties are entirely within the realm of mainstream physics. [Bold text from me.]

The most obvious candidate is a neutron star (see neutron-star), but I also heard about hypothetical exotic stars like the Quark star, but I am unsure how hypothetical those are. I am after exotic matter objects which could (at least theoretically) be somehow observed.
I stumbled upon the Space.com article These 'Strange' Alien Planets May Be Made of Exotic Matter:

However, previous research also indicated that planets made of strange matter could exist and  that scientists might distinguish these planets from planets made of conventional matter via their densities. Normal planets have densities that are no more than 1,870 lbs. per cubic foot (30 grams per cubic centimeter). In contrast, strange planets would typically have densities of nearly 25 million billion lbs. per cubic foot (400 trillion grams per cubic centimeter), Geng and his colleagues said. (For comparison, gold has a density of about 1,200 lbs. per cubic foot, or nearly 19.3 grams per cubic centimeter.)

This adds strange matter exoplanets to my list of exotic matter astronomical objects, isn't it? Are there other exotic matter objects I am missing?


texts: Why do massive rapid-rotating stars move bluewards off the zero-age main sequence (ZAMS)?

texts: I'm reading a well-cited paper by Szecsi et al. 2015 titled "Low-metallicity massive single stars with rotation". The paper can be found here. I was surprised that figure 5 depicted an HRD in which rapid rotators of 500 km/s evolved bluewards on the HRD. I didn't know any star could move anything but rewards when leaving the ZAMS. Other rotators below that 500 km/s move in a more typical fashion. Can anyone explain what is going on here?


texts: Does a star fuse helium to beryllium on the main sequence?

texts:
When a star has finished fusing all its hydrogen into helium, it will then start fusing helium into beryllium and so on and so forth up until iron.

When the star is fusing to beryllium, will the star still be in the main sequence phase and will it at that point start to grow into the red giant phase, or is there no given rule for when it will start growing?


texts: Do differentials in the Earth's density cause the flyby anomaly?

texts:
The flyby anomaly is an occasional, unpredictable, and as yet unexplained discrepancy in acceleration of satellites when they pass close by the Earth.  It causes an increase of up to 13 mm/s during flybys but we have no understanding of it and how to predict it.

The laws of centripetal force predict that due to rotation, the more dense molten material within the Earth might accumulate at the equator, since there it can move further from the axis of rotation.  This will, of course be counteracted by gravity.  Nevertherless, it will be an effect present at the equator more so than at the poles.

This predicts that we should expect to see a flyby anomaly of some magnitude and that it will be correlated with proximity of a flyby to the equator.

Can anybody provide anomaly data and / or correlate it with angle of incidence in order to test this hypothesis?


texts: Apparently two objects in Solar Eclipse image

texts:
These images are taken from a mobile camera of 21 June 2020 solar eclipse.
So the thing surprising me is there are two objects.
I have shown these images to some people. Some says it's just reflection of sun in camera lenses. And some says the blue object is Venus. But we know if that is Venus then crescent should face the sun.
So I want to know what that light blue crescent object is. And if it is the eclipsed Sun then what is the other bright thing?
First image:

Second image:


texts: I have a fits image, and I need to identify an asteroid. What characteristics  do I look out for?


texts:
As far as I understand, a moon is an object in permanent orbit around a planet, dwarf-planet, asteroid, etc. If there was another object permanently orbiting this moon, would that be a moon-moon?

Is there any natural object of such kind that we know of?

If no, why are such objects so unlikely?

Edit: this is question is different from Do moons have moons? because I explicitly ask for a reason. Also, the terminology is included here.


texts: How to compute the sun's (upper) transit

texts:
As detailed in https://physics.stackexchange.com/questions/41450/is-there-a-simple-accurate-formula-for-calculating-transit-times-from-rise-and, the sunrise, and set times can accurately be used to compute the sun's transit. Yet, in some cases, these do not exist, e.g. in Finland in the summer.

Is there an easier and/or more general way to compute the transit time (and only the transit time) given the UTC and observer position (latitude, longitude, height).

The library at http://www.jstott.me.uk/jsuntimes gives sunrise and sunset, but not the transit time. find_transit at riset_cir.c in https://github.com/brandon-rhodes/pyephem/tree/master/libastro-3.7.7 might do this, but seems too general and complicated.


texts: Is it true that open clusters have $E > 0$ and globular clusters have $E < 0$?

texts:
I just need to confirm or refute something I'm believing since a very long time, maybe without real evidence.

Can we distinguish stellar clusters by their total mechanical energy ?

Is it true that all "open" star clusters have a positive energy (i.e. more kinetic energy that potential energy), while all globular clusters are "closed" systems with negative total energy (i.e less kinetic energy than total potential energy) ?

So for example, the Peiades M45 are "open" : they have $E > 0$, while M4, M53 and M80 (globulars) have $E < 0$ ?  Is this true ?


texts: Do stars twinkle when seen from Mars' surface?

texts: Mars' average atmospheric pressure is 0.006 atm (0.088 psi). Is that enough to make fixed stars on Mars' night sky twinkle? Do we know an air pressure or density limit for that?


texts: Constants of three body problem(2 fixed centers)

texts: I was solving two fixed centers problem and got answer in quadratures. After that i want to find constants of motion. First one is full energy - E. Second is $p_{\varphi}$. And third one comes from separation of variables - $\beta$. I want to identify physical sense of this constant. I know how this constants should look like but i can't evaluate them myself. I was searching in different sources and i could find any evaluations. Does anyone know where i can find them?


texts: 2-body orbits expanding in nbody simulation?

texts:
Recently, I designed an n-body engine and tested it with a binary star system. After a minute of running, the orbit began noticeably expanding:

Why is it happening, and is this natural? The source code is based on https://github.com/lemmingapex/N-bodyGravitySimulation.


texts: Total number density of galaxies and problematic expression in practise

texts:
This post comes from physics exchange forum, I have transfered it here, maybe I will be luckier since I have not received answers on the other forum.

Here is the issue :

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice.

From what I saw from my research and into my lectures, I have found the follwing relation which gives the number of galaxies $N$ with magnitude $(m < M)$ (number counts) :

$\text{log}\,N(m<M)\,\propto\,0.6 M + \text{constant}\quad(1)$

$N(m<M)\,\propto\,\text{exp}(0.6 M)\,\text{exp(constant)}\quad(2)$

I don't know what the teacher wants to highlight, i.e by saying that the formal expression of the total number density of galaxies is problematic in practice, given that I even haven't the kind of expression or equation which gives the estimation of galaxy density.

1) Concerning this distribution to use, do you advise me to use a distribution as a function of magnitude, mass or a distribution as function of luminosity (like Schechter) ?

2) I have seen on web the HMF (Halo Mass function) but it seems to be about the dark matter halos : can we count one dark matter halo per galaxy ?

3) But the problem is here is that a dark matter halo is more massive than the galaxy hosted in this halo : how can we deal with this ?

If someone could help me, this would be kind.

PS: I have also to explain why the estimation of galaxies density is difficult and problematic.

4) What are the issues when we want to estimate density of galaxies ?

Concerning the distribution as a function of luminosity, I have found the Schechter luminosity function :

$$N(L)\ \mathrm {d} L=\phi^{*}\left({\frac{L}{L^{*}}}\right)^{\alpha}\mathrm {e}^{-L/L^{*}}{\frac{\mathrm {d} L}{L^{*}}}$$

UPDATE 1: my apologies, $m$ and $M$ represent apparent magnitude and not masses.

But the questions remain about the counting of galaxies and the computation of total density at large scale.

In practise, what are the major problems when we want to estimate this density ?

UPDATE 2: people here not really motivated by this bounty ... feel free to ask for precisions about this post .

For the moment, I am faced to know at which scale I can compute a mean density : have I got to take into account this scale since it seems to have a dependance between density and volume considered, doesn't it ?

Is there a general formula to express this dependance ?

Regards


texts: Do objects lose momentum as space expands

texts: As far as I know, photons' wavelengths can be considered increasing as space expands, making them lose energy and momentum. Does the same apply to physical objects? I understand a photon's speed is the same regardless of the system of reference, and the same does not hold for physical objects, but is it possible to draw any kind of analogy, for instance, would the momentum of an object launched from Earth (neglect non-cosmological effects) be steadily decreasing when looked at from Earth?


texts: How much mass does the Sun lose as light, neutrinos, and solar wind?

texts:
In this answer I estimate the loss of gravitational mass of the Sun to be about 1.3E+17 kilograms per year using $E = m c^2$ and 1360 W/m^2 at 1 AU.

While writing that answer I realized that solar neutrinos also carry away a lot of mass equivalence in the form of kinetic energy. The strongest component seems to be from the first step in the process; deuteron formation from proton–proton fusion.

Am I right so far?

Questions:

How much mass equivalent is lost per year by the sun via neutrinos?
Is my estimate of 1.3E+17 kg for loss per year via electromagnetic radiation close?
Are there any other comparable loss mechanisms? I'm assuming that the solar wind is lower, but I could be way wrong on that!

"bonus points:"

Are these rates likely to have been at least similar 4.5 billion years ago? Or would there have been large changes over this time? ("large" = factors of 2 or so)


texts:
I've recently been reading about the DSCOVR satellite, and I'm confused about it's orbit.
According to NOAA

A million miles away, DSCOVR orbits a unique location called Lagrange point 1, or L1. This point is a gravity neutral point in space, allowing DSCOVR to essentially hover between the sun and Earth at all times.

With it being at L1, it's orbit around the sun is slightly shorter than Earth's. So my uninformed gut feeling tells me it should orbit the sun in a few days less than Earth does, but it clearly doesn't.
How is this object at L1 kept at L1?
Could it be that the gravity of the Earth acting on it in a really convenient way? Or does it use fuel? What fuel? Or is it a property of the L1 point that I have misunderstood that an object will naturally stay there once in position?


texts: How are parachutes usable in other places than Earth?

texts: Mars landings, Moon landings. . . Wherever I see a documentary or still pictures in Wikipedia, there are parachutes in the landing of rovers. Whenever I see them, I ask myself, "How they can it decelerate the speed of rovers, when there is no / thin air?" Something needs to be there to create friction!


texts: What is the standard coordination system used to describe the location at a celestial object approximately due east and at an angle of 10 degrees above the horizon?


texts: Star visibility above horizon map likewise day & light map or moonlight world map

texts:
I am looking for an option to present the given star visibility world map likewise day & night map or moonlight world map like below:

Question: Is it possible to generate a map like this for some particular star or at least the point in the sky defined by celestial coordinates?


texts: What is the exact meaning of vicinity of Black hole or a disk? the same as inside?

texts:
What is the exact meaning of vicinity of Blackhole or a disk? Is this the same as inside the horizon for black holes? If so, why people don't say inside of BH, simply. What is the difference?

I use this phrase actually! and it may seem obvious but not to me, please somebody make it clear. I searched but I couldn't find an exact answer.

Edited: Until which radius from Blackhole I can use this word meaningfully? It's not clear to me its astronomical dimension. "near" is not enough to me. If it could be inside a horizon and also outside of it, it makes a huge difference from point of view of the observer at infinity, as you know, how I can call them by the same name and sometimes I mean one sometimes the other one.


texts: Is there still a red shift when moving perpendicular to the direction of incidence?

texts:
This answer suggests that when there is zero radial velocity... there will be no Doppler shift but it's not likely to be intended as precise and absolute.

I remember reading about something like this in a post here but I can't find it. While to first order there is no frequency shift when moving perpendicular (we wouldn't expect there to be be due to symmetry i.e. which way? Up or down?) I think there is a second order effect which is a Doppler shift associated with the small change in angle due to astronomical aberration.

Question: Is this a thing? If so, what would be an expression for the frequency shift and what is it called? If the magnitude of each effect is $|v/c|$ is it always a red shift and simply

$$\Delta f/f = -\frac{v^2}{c^2}?$$


texts: Do galaxies have a preferred orientation?

texts: This article: http://physicsworld.com/cws/article/news/2011/jul/25/was-the-universe-born-spinning asserts that the universe was "born spinning" and that there is a preferred rotation direction for galaxies in the universe.  My question is whether there is a preferred galaxy orientation in the universe, that is, whether there is a preferred plane for the galaxy disk?


texts: How is the expansion rate of the universe obtained from Baryon Acoustic Oscillations?

texts:
I understand that we can measure the sound horizon from the anisotropies in the CMB. Large galaxy surveys today can construct two-point correlation functions, which reveal peaks at distances equal to today's sound horizon, I think because the BAOs at decoupling essentially created dark matter gravitational wells for structure to begin forming in (correct me if I'm wrong). I'm under the impression that the sound horizon is a fixed physical distance, so its measured comoving distance at any redshift should always be the same. Am I right or wrong in this assumption?

I read some information from a few different sources on this topic which confused me. On Wikipedia: "Peaks have been found in the correlation function (the probability that two galaxies will be a certain distance apart) at 100 h−1 Mpc, indicating that this is the size of the sound horizon today, and by comparing this to the sound horizon at the time of decoupling (using the CMB), we can confirm the accelerated expansion of the universe."

But if the sound horizon is always at the same physical scale, how can we use measurements today to determine the rate of expansion? Do the correlation functions give you proper separations, and from that with different cosmological models we can determine what $H_0$ or $H(z)$ value gives a comoving separation equal to the sound horizon? And is this the same as the BAO signal?


texts: Do galaxies have habitable zones the same as stars do? Say in a galaxy with a very active nucleus producing a lot of heat and radiation, would there be a point at which no star's planets could harbor life do to the effects of the black hole? Also would there possibly be a habitable zone for galaxy clusters? If there were many galaxies with extremely active galactic nuclei condensed closely together would this possibly also hinder the evolution of life?


texts: If the axis of the Earth was through Greenland, what would the north celestial pole be?


texts: Is it possible for a moon to continuously have a side facing its star whilst orbiting a planet?

texts: For example; is there any possibility for the moon to always have a side facing the Sun whilst orbiting Earth? And if so then what would the day cycle be like?


texts: Are there any good DSO API's for python out there?

texts: I was wondering if there was good DSO database API out there for python so I could fetch things like a distance away from Earth or radius etc. I tried looking however couldn't find the exact thing I was looking for.


texts: Do all the objects in the universe exert force on all other objects? Like a type of gravity; also, how much does it decrease as it gets farther away?


texts: Why is a black hole's Hawking radiation temperature inversely proportional to its mass/size?

texts:
As a black hole shrinks in volume and mass, shouldn't its temperature get lower?  Shouldn't it evaporate more slowly?
Naively, (very naively), I think that with a smaller surface area (as per its event horizon), it would have fewer and fewer random quantum foam particles popping up just outside its surface....


texts: what kind of telescope eyepieces is best for viewing planets up close

texts: ok so I am a total amateur when it comes to telescopes and lens, and this is probably a pretty stupid question, but I am trying to find a lens/eyepiece that is good for viewing Jupiter and Saturn up close, and I have no idea what I am looking for, I have an Orion Sky Quest XT8 plus reflector telescope, (not sure if that matters) but if someone knows what I am looking for pls lmk, thx Noelle:)


texts: Is Corona Mass Ejection able to reach the outer planets such as Jupiter and Saturn? If so can it blows away the icy rings or the distance is simply beyond the reach of maybe like Earth it is protected by a magnetic field?


texts: Is the diamond ring effect visible in a 40mm hydrogen alpha telescope?

texts:
I'm going to Nebraska for the 2017 eclipse, and am weighing my options. I'm considering whether to pack my 40mm Coronado PST or to make a Baader AstroSolar filter for my 66mm William Optics apo. I've used the PST for transits and annular eclipses past, but have never seen a total solar eclipse before.

Can I see the diamond ring effect (aka Baily's beads) with my PST, or should I stick with white light?


texts: Identifying cluster of exoplanets between Deneb and Vega

texts:
I noticed an interesting patern of exoplanets.
Is this a known phenomenon?


texts:
How can I obtain ephemeris data which fits to a specific location? Differently phrased: I need ephemeris data of satellites which orbited a specific location. Is there a document where I can look up references?

Right now I am looking in ftp://cddis.gsfc.nasa.gov/gnss/data/daily/2019/ but the subfolders only contain weirdly named broadcast files which don't make much sense to me.

The reason I need these ephemeris files is that I want to test gps-sdr-sim with a USRP I got. I know that those ephemeris broadcast files contain data to every other satellite in orbit (but not in the same file as I understand).

I tried multiple ephemeris files with gps-sdr-sim but none of the data transmitted worked for the location I am in.

Any help would be appreciated.


texts:
"The solar elevation angle is the altitude of the sun, the angle between the horizon and the centre of the sun's disc." - Wikipedia

The meridian is when the sun is directly facing earth and is at exactly 90° overhead at some point along the longitudinal line at a given location.

My question is, if the sun is 93 million miles away and travels overhead at 90° between the Tropic of Capricorn and the Tropic of Cancer, how is it that there are Meridian solar elevation angles reported from relatively nearby cities on Earth of less than 89.99°?

Let's take for instance Calgary, Canada as of today May, 04 2016 -- estimated ground distance from Calgary to where the Sun is at 90° on Calgary's longitudinal line is ~3000 miles. The solar elevation angle (aka "altitude") is reported by the timeanddate website to be 55°! -
TimeandDate

Reversing the angle, using right triangle math:

The base angle, our solar elevation angle, of a right angle triangle is:
    "solar elevation angle"= arctangent (h/a)
    89.998151749049 = arctangent (93000000/3000)

So using right triangle math alone with the presumption of the Sun being 93 million miles away and base distance of 3000 miles we get an expected solar elevation angle of 89.998151749049°.

That makes sense, since if something is 93 million miles away from us and you think of it as a triangle, then the distance between any place on earth and the location on earth where the sun is at exactly 90° overhead -- you should expect the other angle of the triangle to be at near 90°, connected by a relatively tiny sliver of distance in a much elongated triangle to the center point of the distant sun.

If we go the other direction and leave out the presumption that the sun is 93 million miles away. We take h as unknown, we know the reported solar elevation angle of 55°, we can again use right triangle math:

    Let a=3000, solar elevation angle=55°
     h = a * tangent ("solar elevation angle")
     4,284.4440202263 = 3000 * tangent(55°)

So according to my calculations, based on the advertised solar elevation angle of 55° at Calgary, Canada on May, 04 2016 at Meridian, at a ground distance of 3000 miles from where the Sun is at exactly 90° overhead --  the Sun is actually only 4,284.4440202263 miles high!

** Update **: The math would work out for a sphere Earth with a radius of 4000 miles, using the exact solar position and adding  corrective angles due to a presumption of a spherical curvature:
    Calgary, Canada: 51°03′N 114°04′W
    Location of Sun at Calgary Meridian: 16°23′N 114°04′W
    Distance = 2395.386 miles
    Radius of Earth=4000 miles
    Arc distance = 4790.772 miles
    Width of arc = 4509.52059 miles
    Angle to center of the Earth=68.622754110849°
    68.622754110849°/2=34.31137705542450°

This math utilized calculations that may have had a bit less acurracy but to make it work you need to add a bandaid padding of approximately 34.31137705542450° to Calgary,Canada's advertised angles. If you do you arrive at 55°+34.31137705542450° = 89.31137705542450° which is close to what I would expect to see. If, we are truly living on a Flat Earth, then the calculations would be telling us that the Sun height is only 4,284.4440202263 miles high and we can use simple trignometry.
Arc Calculator
Coordinate Distance Calculator

If the math is difficult for you, you can check with an online triangle calculator such as: Ke!san


texts: Can an event horizon form with the observer already inside?

texts: I am on a planet orbting a star around the center of the milky way (assume no supermassive blackhole there). By some mechanism, mass starting from the outer part of the galaxy gets pushed inward until the schwarzchild radius is reached for that mass (.25 ly according to Wikipedia). I'm under the impression that an event horizon would form from the perspective of someone looking at our galaxy from the outside. But how would it look like from my perspective at the center of the galaxy? Would I view everything as normal? I know things would start collapsing with so much concentrated mass, but would I know I was inside an event horizon?


texts: Distance and orbital period for terrestrial binary planets

texts: This article, http://phys.org/news/2014-12-binary-terrestrial-planets.html, suggests binary planets could orbit each other at a distance of only three planet radii. For two earth-like planets, that is approximately 12,000 miles. Does this distance seem accurate? How long would it take for these planets to orbit their barycenter?


texts:
Can any one explain to me What will happen to Earth if its has two moons?

Same Distance & two sides of earth (North & south)?

Will it be good for humans or bad?


texts: Which are the consequences of Uranus' high rotation axis angle?

texts: The rotation axis of Uranus is 90 degrees away from vertical, there are some theories about its reason.. But which are the consequences of this? Which are the unique features of Uranus that happened because of it?


texts: I am debating if I should buy a 2x Barlow Lens. My telescope, a go-to Meade star navigator, has an aperture of 114mm, focal length of 1000mm and I currently have a 9 and 26mm eyepieces. I want to see Jupiter and Saturn better specifically, but I wonder if such a lens is worth it.


texts: When did "resilvering" large telescope mirrors actually refer to aluminization, and why was it necessary?

texts:
This answer nicely summarizes the general picture of historical use of silver for mid to large size telescope mirrors, and the shift to aluminum once good quality vacuum pumps and chambers suitable for this process were available. A freshly evaporated, atomically clean aluminum surface will can sometimes violently react with oxygen, so this has to be done under high vacuum, and vented carefully with inert gas first.

When did the term "resilvering" generally start to actually refer to aluminum coating, and not silver? Was that actually the case by the 1960's or 1970's?

From what I remember, evaporated aluminum will form a protective oxide layer ("native oxide") of the order of 10A thick within seconds or minutes that is self limiting - no further oxygen can penetrate this layer.

If this is so, why was re-silvering actually necessary? The cost, risk, and downtime associated with removing the primary and other mirrors every few years sounds like something you wouldn't do unless absolutely necessary.


texts: The elemental make-up of Jupiter is about entirely hydrogen and helium, along with a very small fraction of the atmosphere being made up of compounds such as ammonia, sulfur, methane, and water vapor. These elements are dominant in stars, so how likely is it that Jupiter was a companion to our sun, but failed to ignite?


texts: Why do some neutron stars become pulsars?

texts:
I suspect this may be an open problem in astronomy, but I have to ask anyway.

There doesn't seem to be any large-scale difference in the formation of pulsars from the formation of "normal" neutron stars. Yet pulsars end up emitting electromagnetic radiation, while others - "radio quiet" neutron stars - do not.

Why do are some neutron stars formed as pulsars, while others are not?

This is partly based on What causes a star to become a pulsar?, but the answer does not cover this part.


texts: Did the Moon have a substantial atmosphere in the past?

texts:
The layer of gas surrounding the Moon is very thin. It is a surface bound exosphere, where particle-particle collisions are rare.
In the past, the Moon was more geologically active, with eruptions filling craters with magma and whatnot. Eruptions often come with out-gassing. Was this enough to give the Moon a thicker atmosphere, or was this gas immediately stripped away?
Did the Moon have an actual atmosphere at any time in history? If so, when did the lunar atmosphere reach the pressures we observe today?


texts: Is seeing the apollo moon Landers via earth telescope that hard?

texts: There are interferometers, which are combining multiple telescopes together, with 0.001 arcsecond resolution. With changes, couldn't these observe the Landers? Is the reason why nobody is seeing the Landers related to effort and not actual technical limitation? Besides atmosphere interference, brightness, time use and other general issues.


texts: Questions related to the reflection of the moon

texts:
What would happen if we painted the whole moon with black, will it still reflect light, or it will become invisible?
What would happen if we painted the whole moon with silver paint or make its surface shiny?


texts: Why is there a matter/anti-matter asymmetry in the universe?

texts: After the big bang occurred what biased the formation of particles over anti-particles? Why are particles more common than anti-particles?


texts: What is the main source of opacity in sunspots?

texts:
In the first version of my answer to Are sunspots vertically displaced from the surrounding photospheric plasma?, I naively used Kramer's law to attempt to show an inverse relationship between opacity $\kappa$ and temperature $T$ due to bound-free or free-free absorption in a region with density $\rho$:
$$\kappa\propto\rho T^{-7/2}$$
Rob Jeffries pointed out that this might not be accurate in the photosphere, even though it works well deeper inside most stars (though for high-mass stars, Thomson scattering dominates and $\kappa$ is approximately constant). After some more reading, I concluded that this was right. At lower temperatures, interactions with ionized hydrogen mean that $H^-$ is the major contributor to opacity, and it follows a law
$$\kappa\propto\rho^{1/2}T^9$$
I believe the transition of dominance happens around $\sim10^4\text{ K}$, meaning that $H^-$ opacity should be the major contribution in the stellar photosphere.

At even lower temperatures, however ($\sim4000\text{ K}$), molecular scattering can be important (see Tennyson et al. (2007)). While this is not necessarily important at all points in the stellar photosphere, I think that it should be important in sunspots, which can have temperatures in this range.

What is/are the major source(s) of opacity in sunspots? Does molecular scattering dominate, or is $H^-$ still important? If both are important, to varying degrees, how can the mean opacity be calculated?

My knowledge of stellar atmospheres and atmospheric models is limited, so I apologize if this comes down to being a basic question.


texts: Physical properties of the stellar corona?

texts:
I am after the basic physical properties of the corona of a star, here some questions for which I would like to have an answer, even only for a subset of them:

I know some things about the corona of our sun, but how typical is the solar corona?
Is there any rule of thumb for the average dimensions of the corona, possibly depending on the mass of or other parameters of the star? Is it possible to estimate to at least an order of magnitude how thick a shell around a star would be which would be most likely be filled with plasma?
What are the main constituents of the coronal plasma?
If I intend to model the corona as an homogenous shell (which definitely is a strong assumption), what are the ranges of the number densities of the abundant particles?

Related

In the closest stars, can you see the stellar corona?
What is the dielectric constant of a star?


texts: So this may be an incorrect assumption, but from my knowledge solar systems are heliocentric and there are always suns. (I realize to be a solar system there must be a sun semantically.) But it seems to me that planets and stars are just bodies of mass, so are there cases of solar systems where the star is just one of the bodies of the system and orbits around a super massive planet? And similarly are there 'solar systems' without stars, where there are a group of planets orbiting a massive planet at the center of the system?


texts: What exactly are "H30α and He30α images"? (ALMA)

texts:
I ran across this paper's title η Carinae: high angular resolution continuum, H30α and He30α ALMA images (arXiv) and see that the body of the paper also mentions "H40α, H30α and H29α".
What notation is this, in which contexts is it used, and what exactly to the numbers and greek letters denote? (I'm assuming that H and He are elements).


texts: Recommend a radio calibration target for amateur radio telescope

texts:
I have a small antenna and radio system that can receive frequencies between 1MHz and 1GHz.  The system has a discone antenna. I plan to do some aperture synthesis based on the daily rotation of the Earth that will allow me to resolve a lune with a width of about one degree.
What objects and specific frequencies should I observe to test to see if the system and algorithm is functioning properly?


texts: What are the most popular computer programming languages in observational astronomy?

texts:
I would like to work in the field of observational astronomy (not so much theoretical astronomy).

I currently know two computer programming languages: C# and MATLAB.

I have read that computer programming is absolutely essential to every astronomer, and that it takes a very significant part of the time of the work of most astronomers.

Therefore I would like to learn a few more programming languages. Hence my question:

What are the most common programming languages in observational astronomy?


texts: How to set in Liberator 3 the reverse color scale? I would like to have stars black and the surroundings white. Or could you recommend me another program that has this ability?


texts: Correlation between supermassive black hole properties and galaxy rotation speed

texts:
Is there any correlation between the size or spin - or any other properties - of a galaxy's central supermassive black hole and the speed of galaxy rotation (which I think would also be proportional to the calculated amount of dark matter in the galaxy [?])?
Thanks!


texts: Around what apparent magnitude can the naked eye observe an object during full moon

texts: For a very rough guideline using healthy/corrected eyes adjusted to the dark, around how bright should an object be to expect it to be visible?


texts: Accuracy of predicting an asteroid strike in 1987

texts:
Good People,

I am writing a short story, and I wanted to include a section wherein a family is told that an asteroid is going to strike their home. The thing is, I would like to set the story in 1987, but I'm not sure if there was the capability of that kind of prediction back then. To be honest, I'm not even sure that kind of accuracy is possible now! But I will allow myself some creative freedom in that area, but I don't want to stretch it so much that someone would keep thinking to themselves, "Really, in 1987 they could predict that?" Would such a thing have been possible then? If not then, what about sometime in the 90s?

While I have your eyes, is there a specific size that an asteroid would have to be in order to just destroy one home (or a couple on the side of the home).

Thank you for any insight and info! I appreciate the resources here. I did some digging around before asking this question: nothing seemed to be particular to my frame of reference. If it exists, please let me know.


texts: What's the bleeding-edge model for how Population III stars are born and evolve?

texts:
I'm trying to do a brief literature review to find out what the best current models for population III stellar evolution are. I was hoping that someone with more expertise in the area could perhaps summarise or point me in the right direction.

This post on physics.SE is a great intro, but it's now over 4 years old.

Recent observations (see here and here) suggest hints of these kinds of stars at relatively low redshift (z ~ 6) - does this provide significant tension on current models? Have there been other observational hints?


texts:
I understand that Kerr Black Holes are purely theoretical and have not been found anywhere in the Universe. However, they are theorized for possible means of time travel.

I am attempting to prove in a research paper that controlled time travel is statistically improbable for the human species. Therefore if scientists have evidence that there does not exist a black hole within a distance of X from Earth, and it is statistically improbable for the human species to ever be able to travel a distance of X, I can rule out time travel by means of kerr black holes with a significant degree of certainty.

However, I cannot find this "X" threshold, which is why I am asking this question. Have scientists verified that there are no kerr black holes within a distance of X from earth for some X?


texts: Will Neptune be visible with the naked eye if I am standing on its satellite

texts:
Assume that I am standing on one of the many satellites of Neptune. Will I be able to see the planet with my naked eyes (without any sort of visual aid).

If I was to stand on Moon I would be able to see the Earth because the light from Sun is reflected off Earth into my eyes and I am able to see Earth. In case of Neptune, the distance between Neptune and the Sun is so great that the sun would look like any other star (a bit brighter). In that case if I am looking at Neptune would I be able to see it ?

What about the other gas giants ?


texts: How close must a full moon be to become a supermoon?

texts: According to this answer there is a scale of super moons depending on how close the full moon(henceforth anti-sol) is to perigee. So how close to anti-sol does the moon's perigee need to be to be referred to as a "super moon"?


texts: How to calculate the stellar visibility?

texts:
Please could you let me know a (basic) formula for calculating at what time a star would become visible of an evening in clear weather conditions.  I presume the formula would involve the stars apparent magnitude, the location of the observer, and how far the sun has dipped below the horizon (so, the sky's limiting magnitude).  Working with these three parameters, I guess there must be a simple enough formula, however I cannot find it!  Could you help with this?  For example, assuming clear weather and good eyesight, how could I calculate at what time the star Deneb will become visible on the 1st May
2019 for a rural location?
I am asking this as I want to create an art piece where something happens each time a new star (or stars) becomes visible around twilight time....


texts:
Limits of space telescope? links to the video 4 Future Space Telescopes NASA wants to build and that page links to The New Great Observatories. These cover the four space-based instruments proposed by study groups for the 2020 Decadal Survey on Astronomy and Astrophysics.
The first one mentioned is Habitable Exoplanet Observatory or HabEx which will be two spacecraft in a halo orbit around Sun-Earth L2 separated by perhaps 60,000 kilometers. One is a telescope equipped with an internal coronagraph and active wavefront modulation, the other is a passive starshade which is an occulting disk with diffraction-reducing apodization (petals).
I can't understand how these two mechanisms for reducing/removing light from a star from the final image so that exoplanets around that start can be imaged.
Question: How do HabEx's internal coronagraph and external starshade work together and complement each other? What is it that each can do that the other can't?


texts: What is the origin of the term "plane of the spirit level"?

texts:
I have heard the term "plane of the spirit level" e.g. the sun's shadows is X degrees to the plane of the spirit level to signify the surface of the earth and I see it is a common term used in surveying and as an instrument to show if there is a curvature.
My question is: where is this term coming from?


texts: A friend has a vintage telescope with a broken lens, I asked him to send me its catalog so I can see if I can buy him a new one, he gave me what looks like a Russian catalog, it seems a soviet telescope, I can't read it, I hope you could tell me what it is. I want to search for its lens using English characters.